When functioning normally, the heart produces rhythmic contractions and is capable of pumping blood throughout the body. However, due to disease or injury, the heart rhythm may become irregular resulting in diminished blood circulation. Arrhythmia is a general term used to describe heart rhythm irregularities arising from a variety of physical conditions and disease processes.
An abnormally fast heart rate is designated tachyarrhythmia. Tachyarrhythmias may originate in either the atria or the ventricles. Tachycardia is a term generally used to describe cardiac rhythms that are rapid, but relatively organized. Conversely, fibrillation is characterized by rapid, chaotic, and disorganized heart rhythms. Tachycardia and fibrillation may affect either the atria or the ventricles.
Pacemakers have been used as an effective treatment for patients with serious arrhythmias such as bradycardia, a condition characterized by an abnormally slow heart rate. Pacemakers typically include circuitry to sense electrical signals from the heart and a pulse generator for delivering a series of low energy electrical stimulation pulses to the heart. Leads extending into the patient's heart are connected to electrodes that contact the myocardium for sensing the heart's electrical signals and for delivering stimulation pulses to the heart. The pace pulses may be intermittent or continuous, depending on the patient's metabolic demand. The pace pulses are timed to assist the heart in producing a contractile rhythm that maintains cardiac pumping efficiency.
When a pace pulse produces a contractile response in heart tissue, the contractile response is typically referred to as capture, and the electrical cardiac signal corresponding to capture is denoted the evoked response. A pace pulse must exceed a minimum energy value, or capture threshold, to produce a contraction. Detection of the evoked response may be used to verify that the pace pulse has produced capture of the heart tissue.
Cardiac rhythm management systems may include both a pacemaker and a cardioverter/defibrillator. An implantable cardiac cardioverter/defibrillator (ICD) typically monitors cardiac activity and delivers high energy electrical stimulation to the heart to interrupt a tachycardia or fibrillation condition. In general, the ICD continuously monitors cardiac activity by analyzing electrical signals, known as electrograms (EGMs), detected by endocardial sensing electrodes. ICDs are generally capable of diagnosing the various types of tachyarrhythmias discussed above, and then delivering an appropriate electrical stimulation therapy to the patient's heart to terminate the diagnosed arrhythmia.
In general, atrial tachyarrhythmias are chronic conditions that are non-life threatening, because the atria only aid in the movement of blood into the ventricles, where the major pumping action of the heart occurs. Conversely, ventricular tachyarrhythmia is a life-threatening event because the heart's ability to pump blood to the rest of the body is seriously impaired if the ventricles become arrhythmic.
In the treatment of chronic cardiac conditions, such as atrial tachycardia or fibrillation, the patient is typically conscious and can feel the electrical stimulation applied to the heart. Thus, it is desirable to reduce the energy of the electrical stimulation for treating arrhythmias, particularly chronic atrial arrhythmias. For both atrial and ventricular tachyarrhythmias, the relative organizational state of the tachyarrhythmia may be related to the amount of energy needed for successful defibrillation therapy.